As global demand on hydrocarbon reserves has continued to increase, more efficient utilization of petroleum and gas reserves has become an important complementary strategy to the development and deployment of sustainable energy generation.
In particular, alkene (olefin) production is critical for the polymer and chemical industries and is widely utilized as intermediates in the production of transportation fuels. Current olefin production is generally accomplished by thermal cracking of alkanes at high temperatures, to the olefin and hydrogen, and catalytic dehydrogenation with Pt nano-particle or Cr oxide catalyst technologies at temperatures above about 600° C. where equilibrium favors high alkane yields. For alkanes with three or more carbons, thermal cracking results in mixtures of C—C and C—H cracked products. Propane, for example, produces propylene, ethylene, hydrogen, and methane. Because of the low olefin yields by thermal cracking for C3 and higher hydrocarbons, catalytic conversion processes are often favored. While propylene selectivity is higher for catalytic dehydrogenation of propane than thermal cracking, increasing the propylene selectivity, i.e., reducing the C—C cleavage reaction in favor of the dehydrogenation, remains an important catalytic goal that increases the overall process efficiency by requiring less separation of the products. With catalytic dehydrogenation, there is also deposition of carbon (“coke”) on the catalyst surface leading to rapid loss of activity, often in a few hours, thus requiring frequent regeneration and expensive process designs. As a result, improved catalytic materials with higher selectivity, rate, and lowered coke production is an important goal to improve alkene production.
Natural gas production and its reserves in the United States provide a valuable natural resource for energy security. The domestic production of natural gas has increased by approximately one trillion cubic feet per year over the past decade due primarily to recovery from oil shale wells. Recent methods for harvesting natural gas from shale gas deposits decouple natural gas production and cost from those of petroleum. A wide variety of approaches to methane coupling have failed to yield commercializable technology despite intense interest over essentially the whole history of catalysis research. The problems are well-documented and understood, with the principal problem being the necessity of high temperatures for favorable thermodynamics and the kinetic instability of reaction products relative to methane. Heat management is a problem, as dehydrogenation endothermic need efficient means to provide heat—basis for reactor choice. Further, conversion equilibrium is limited, which can require altering pressure or temperature to try to drive conversion. Further, even for catalysts that exhibit acceptable performance initially, catalysts experience high coking levels. This is exacerbated when temperatures are increased to increase conversion, as coking and side reactions (such as cracking and coke formation) increase.
Of particular interest is the Fischer-Tropsch (FT) process, an indirect methane conversion route, which first converts methane to syngas followed by FT synthesis to produce gasoline and diesel. As an alternative to the indirect conversion route, methane can be converted to a liquid fuel by a number of reaction processes including (a) non-oxidative coupling of methane (NOCM) to produce ethylene followed by an oligomerization process to yield aromatics or longer chain linear alkanes/alkenes; (b) oxidative coupling of methane (OCM) in which methane is reacted with a sub-stoichiometric amount of oxygen to produce ethane or ethylene as the primary products, and CO2 and H2O as secondary products, which can also be coupled with an oligomerization process; and (c) selective oxidation to produce methanol followed by a process such as the Mobil “methanol-to-gasoline” process. Conceptually, NOCM should have an economic advantage over OCM or selective oxidation since it does not require the use of expensive oxygen. However, to date, no NOCM process has progressed to a commercial stage. Despite these challenges, methane activation remains an attractive problem.